The present invention is directed to an improved process for forming conductive through-holes through a dielectric, i.e., an insulating layer, sandwiched between two conductive layers. The process is particularly useful in formation of printed circuit boards.
U.S. Pat. No. 3,969,815 discloses a process for providing an electrical interconnection of two metal layers positioned on opposite sides of a substrate. A hole is initially drilled or bored through the two metal layers and the intermediate insulating layer. The hole in the insulating layer is enlarged by a selective etching process which only etches the insulating layer to form an enlarged annular hole in the insulating layer which undercuts the metal layer portions. Thereafter these overhanging metal portions on opposite sides of the insulating layer are deformed by application of pressure to contact or almost contact one another. The deformed metal portions are coated by galvanic metal which is overcoated by a thin layer, preferably tin, to form a conductive path.
U.S. Pat. No. 4,472,238 discloses forming a conductive through-holes by plasma etching holes completely through a dielectric sandwiched between conductors and by deforming at least one conductor which has been undercut during the etching.
U.S. Pat. No. 3,557,446 discloses a method of forming printed circuit board conductive through-holes using a hydraulic medium such as a rubber pad to extrude, explode or rupture a conductive foil sheet over a hole in a nonconductive substrate in formation of a conductive through-hole in the nonconductive substrate.
U.S. Pat. No. 4,319,708 discloses formation of an electrically conductive path through an insulating plate using dies to distort conductive sheets into holes in the insulating plate.
British Pat. No. 1,010,335 discloses a method of forming embossed edges around holes in a metal plate using elastic material to uniformly distribute pressure.